In manufacturing of very large scale integrated circuits, photolithography by excimer laser is one of the most important processes. At present, the primary laser source used for photolithography device for semiconductor integrated circuits manufacture is an excimer laser of Argon Fluoride (ArF) or Krypton Fluoride (KrF) with output wavelengths of 193 nm or 248 nm, respectively. In a photolithography device, a large number of deep ultraviolet optical elements are utilized, including reflective optical elements, transmissive optical elements, attenuating optical elements and the like, to shape, transmit and control the laser beam with wavelength of 193 nm or 248 nm. Among these optical elements there is a large number of elements for the imaging objective lens. The manufacturing of these elements for the imaging objective lens is difficult and expensive. On the other hand, when these deep ultraviolet optical elements are under long-term irradiation of excimer laser beam at wavelength 193 nm or 248 nm, color centers or other physical or chemical processes may occur within these elements, which leads to a slow degradation of their optical property until a disastrous damage occurs and the lifetime of these optical elements ends. Previous researches show that the degree of degradation of the optical property of these optical elements is directly related to the power density of the laser beam irradiation (P=E/τ, wherein P is the power density, E is the energy of the pulse, and τ is the pulse width). The higher the power density, the faster the degradation, and the shorter the lifetime. On the other hand, in order to enhance the yields of the photolithography devices, energy of the output pulse of excimer laser for photolithography is continuously increasing in recent years, from 5 mJ at the early stage to 10 mJ and 15 mJ at present. However, the pulse width of the output pulse of the laser is basically not changed, and is still around 20 ns. The increase of the pulse energy may greatly shorten the lifetime of the optical elements used in the photolithography devices, especially that of the expensive optical elements of the imaging objective lens.
One effective means for increasing the yields of photolithography devices with the increasing output pulse energy of the excimer laser but without decreasing (but on the contrary even extending) the lifetime of the optical elements is to stretch the width of the output pulse of the excimer laser. For example, if the output energy of laser pulse is increased from 5 mJ to 15 mJ and at the same time the pulse width of the laser beam is extended from 20 ns to 60 ns, the power density of the laser beam in the optical photolithography system is kept the same, so it does not affect the lifetime of the optical elements. On the other hand, if the pulse width is extended to 120 ns, the power density is reduced to be one half, the lifetime of the optical elements may be effectively extended and the use cost of the photolithography system can be reduced.
Since the output pulse width of the excimer laser is kept to be basically constant (around 20 ns), a pulse stretching device can be utilized to extend the pulse width. The method for stretching the pulse width of the excimer laser is mainly based on beam splitting by splitting elements and time delaying by optical resonators. In this method, one pulse is divided into overlapping of a plurality of pulses with different time delays, overall the pulse is extended in duration. A conventional pulse stretching device generally utilizes one beamsplitting element and one confocal resonator to achieve a ratio of pulse stretching (the ratio of the width of the stretched output pulse to that of the original input pulse) of around 2.5-3.0. If a higher ratio of pulse stretching is required, a plurality of pulse stretchers (in series) are utilized. For example, a Chinese Patent application with an application number 201010178364.9 and entitled “Pulse stretcher with a decreased power density within optical components” discloses a pulse stretching device based on one beamsplitting element and one confocal resonator constituted of two spherical reflective mirrors. The basic configuration of this pulse stretching device is identical to that of the conventional pulse stretching device except for that a folded reflective mirror is added to implement a compact configuration and a beam expander is added to decrease the probability of damaging the optical elements in the device. Other possible alternatives mainly include configuration modification of the optical resonator, for example a resonator having three reflective mirrors (U.S. Pat. No. 7,035,012 “Optical pulse duration extender”), a resonator with four reflective mirrors (U.S. Pat. No. 7,415,056, “Confocal pulse stretch”), and so on. In order to increase the ratio of pulse stretching, Burkert et al. [A. Burkert, J. Bergmann, W. Triebel, and U. Natura, “Pulse stretcher with variable pulse length for excimer laser applications”, Review of Scientific Instruments 81, 033104(2010)] put two pulse stretching devices in series. In other methods for further increasing the ratio of pulse stretching, for example, a retro-reflective mirror is employed to reflect the stretched output beam back into the pulse stretching device for twice stretching so to implement a quadratic stretching (US2006/0216037: “Double-pass imaging pulse-stretch”). In these methods and devices, the pulse stretching systems based on one beamsplitting element and one optical resonator either have a lower ratio of pulse stretching, or the stretched output pulses have a poor temporal waveform. The methods and devices based on two pulse stretching devices in series or quadratic stretching by utilizing a retro-reflective mirror lead to a larger energy loss since there is reflection loss for every reflection (e.g. the typical reflectivity of a reflective mirror at 193 nm is around 98%).